The invention relates to an exhaust gas turbocharger for an internal combustion engine including a compressor in the intake duct and a turbine in the exhaust duct. The turbine includes a housing with a flow passage provided with a variable vane structure for controlling the exhaust gas flow into the turbine.
DE 43 30 487 C1 discloses an exhaust gas turbocharger having radial and semi-axial exhaust gas flow to the turbine wheel. A variable turbine geometry is provided in the form of a guide structure having variable guide vanes for the variable setting of the flow entry cross section of the turbine. By changing the flow entry cross section, varying levels of exhaust gas counter-pressures can be achieved in the section between the cylinders and the exhaust gas turbocharger, as a function of the operating state of the internal combustion engine. As a result, it is possible to control the output of the turbine and the output of the compressor as needed. In the powered operating mode, the efficiency of the exhaust gas turbocharger can be improved in this manner. In an engine braking operating mode, high engine braking performances can be achieved as the turbine geometry is converted to a shut-off position that reduces the flow cross-section into the turbine, so that the exhaust gas back pressure between cylinder outlet and turbine is increased and the piston has to perform compression work against the high excess pressure in the exhaust gas line during the compression strokes.
A comparable exhaust gas turbocharger is also disclosed in U.S. Pat No. 5,758,500.
Exhaust gas turbochargers are further known which have a double flow turbine housing, as for example those known from DE 195 40 060 A1, which have two flow passages, divided by a partition, for feeding the exhaust gas to the turbine wheel. The double-flow form of the turbine housing is advantageous for impulse charging of the exhaust gas turbocharger, in which the pressure wave formation, which is dependent on the ignition sequence of the individual cylinders, is utilized to increase the charger performance.
A double-flow turbine housing is also known from the document entitled xe2x80x9cLecture manuscript for the faculty of the internal combustion enginesxe2x80x9d, Volume II, December 1985, Aachen Technical University, page 281.
Publication U.S. Pat. No. 4,449,731 shows an exhaust gas turbocharger into whose flow passage inserted an axially adjustable spring-loaded annular piston is disposed, whose position is adjusted to an equilibrium state between the spring force on the one hand and the exhaust gas back pressure on the other hand. The piston is a passive control element, whose position depends on external forces acting upon it. The volume of the flow passage, however, cannot be selectively set in the arrangement of U.S. Pat. No. 4,499,731, so that a selective switch over between impulse pressure charging and back pressure charging of the turbocharger cannot be performed. In addition, the flow conditions in the flow passage are changed as a result of a change in position of the control piston, which may result in a decline in performance in certain operating ranges of the internal combustion engine.
Publication DE 42 00 507 C2 discloses a variable leaf spring in the flow passage of a turbo machine. The position of the leaf spring is adjustable by an adjustment device. A change in position of the leaf spring changes the volume and the form of the flow passage. As a result, however, the flow conditions may also change in an undesirable manner.
By way of prior art, reference is further made to publications DE 197 27 140 C1 and EP 0 884 454 A1, which also relate to turbochargers.
It is the object of the present invention to improve further the utilization of the exhaust gas energy of an internal combustion engine having an exhaust gas turbocharger whose turbine has a variable turbine geometry.
In an exhaust gas turbocharger for an internal combustion engine, in which a compressor is disposed in the intake duct and a turbine is disposed in the exhaust duct, and wherein the turbine has a turbine wheel in a turbine housing with at least one inlet flow passage for the introduction of exhaust gas which flow passage is provided with a variable turbine geometry for the variable setting of the effective turbine inlet flow passage cross-section, the variable turbine geometry being adjustable between a shut-off position that minimizes the turbine inlet flow cross-section and an open position that maximizes the turbine inlet flow cross-section, a gas-collecting chamber is provided which is in communication with the inlet flow passage in the turbine housing, the total volume of flow passage and the gas-collecting chamber being adjustable by an actuator movably disposed in the turbine housing as a function of the operating state of the internal combustion engine.
The gas-collecting chamber of variable volume fulfills a balancing or compensating function, because the gas volume available within the turbine housing for the pressure transmission of the exhaust gas can be adjusted as necessary to the current situation. This makes it possible to create conditions that permit both impulse pressure charging and back-pressure charging of the exhaust gas turbocharger, both with high efficiency, as a result of which improved energy output is obtained over a wide operating range.
In the case of impulse pressure charging, where residual energy of the exhaust gas contained in the combustion chamber is abruptly transmitted to the turbine wheel when the outlet valves are opened, it is expedient for optimum effectiveness of pulse transmission, to keep the total volume in the turbine which is available for the transmission of the pulse as low as possible. For this purpose, the flow passage in the turbine housing is connected with the gas-collecting chamber of variable volume, the volume of which is reduced if boost pressure charging of the exhaust gas turbocharger is desired, so that the total volume within the turbine housing is likewise reduced and the conditions for the pulse transmission to the turbine wheel are advantageous.
In back-pressure charging, by contrast, the exhaust gases of all the cylinders are collected and fed to the turbine, ideally under approximately constant pressure, for which an enlarged total volume in the turbine housing is expedient. The equal application thus achieved results in more efficient performance of the turbine.
The enlargement of the total volume can be achieved by incorporating the gas-collecting chamber volume, the connecting aperture between flow passage and gas-collecting chamber merely being opened when desired but the two volumes otherwise being kept constant. According to another advantageous embodiment, the volume is changed by continuous enlargement of ht gas-collecting chamber volume, the gas-collecting chamber either being in permanent communication with the flow passage or being connected to the flow passage so as to be capable of being switched on and off.
Especially at low mass throughputs through the turbine, at which only a low exhaust gas back-pressure is built up, which would result in an energy output insufficient for impulse pressure charging, the volume of the gas-collecting chamber is reduced, so that impulse pressure charging (utilization of the exhaust gas pressure pulses) can be effectively used. At higher mass throughputs, however, with an enlarged volume, high exhaust gas back pressures with correspondingly high turbine performances can be achieved via impulse pressure charging.
According to an advantageous embodiment of the invention, the situation-dependent setting of the volume of the gas-collecting chamber is achieved via an actuator, which forms a component of the variable turbine geometry for the variable setting of the turbine wheel entry cross section. This actuator therefore has, in addition to the function of setting the cross-section geometry, that of adaptively setting the volume of the gas-collecting chamber or the total volume in the turbine housing. In this way, two functions can be performed with a single component and a simplification of design is achieved thereby. The actuator is disposed, relative to the gas-collecting chamber, so that at least one actuator positions exists in which the gas-collecting chamber is connected to the flow passage. It may be advisable here both to provide only one position of the actuator in which the gas-collecting chamber communicates with the flow passage, whereas it is otherwise isolated from the flow passage, and to permit a continuous or virtually continuous adjustment of the gas-collecting chamber volume as a function of the actuator position.
Preferably, a wall or a plurality of walls of the gas-collecting chamber are bound by the variable turbine geometry or the actuator of the turbine geometry, so that an adjustment of the turbine geometry automatically results in the enlargement or reduction of the gas-collecting chamber volume.
It is advantageous, in the case of a turbine that is provided with a radial and a semi-axial turbine wheel inlet cross section, to arranged the gas-collecting chamber adjacent to the radial turbine wheel inlet area, especially between the radial and the semi-axial turbine wheel inlet flow passages which provides for a space-saving design that can easily be produced. The actuator, preferably formed as an axially displaceable annular sleeve, is axially adjustable between a shut-off and an open position. The gas volume of the gas-collecting chamber and flow passage have a minimum value in the shut-off position. In the shut-off position, advantageously, the radial turbine wheel inlet cross-section is closed, so that the exhaust gas fed to the turbine can act on the turbine wheel only via the semi-axial turbine inlet flow passage. In the open position, by contrast, the turbine wheel is supplied with gas both via the semi-axial and via the radial turbine inlet flow passages. The shut-off position, which is associated with minimization of the gas-collecting chamber, is expediently provided for impulse pressure charging in which the internal combustion engine is operating, the open position however being provided for back-pressure charging.
Preferably, a plurality of separately formed flow passages are formed in the turbine housing, and lead to the semi-axial turbine wheel inlet area. They act upon the turbine wheel by segments, viewed over the circumference, as a result of which exhaust gas mixing between individual flow passages is prevented. In the open position of the variable turbine geometry, the flow passages communicate with the gas-collecting chamber, via which exhaust gas can be fed to the turbine wheel via the radial turbine wheel entry cross section.
In the shut-off position, by contrast, the volume of the gas-collecting chamber is reduced and the radial turbine wheel entry area is blocked or reduced to a minimum. The flow passages are separated from the gas-collecting chamber via a partition, including a connecting aperture. The cross-section of the connecting apertures can be enlarged or reduced by the actuator of the turbine geometry.
The invention will become more readily apparent from the following description of preferred embodiments on the basis of the accompanying drawings: